The structure property correlation of different commercial grades of high-impact polystyrene (HIPS) with varying polybutadiene rubber (PBR) content has been investigated. The average molecular weights of polystyrene, rubber content, gel content, swelling index, degree of grafting, morphology of rubber, and rubber particle size distribution were characterized and correlated with the mechanical properties of HIPS. The dynamic light scattering and transmission electron microscopy techniques were employed to characterize the loss of submicron tiny PBR gel particles. The obtained results reveal the fact that submicron tiny PBR gel particles that are lost during gel estimation by ultracentrifugation process lower the gel content and rubber efficiency, which significantly contribute to the overall impact performance evaluation of the final HIPS product.